This invention relates generally to digital storage oscilloscopes, and in particular to a digital storage oscilloscope with an automatic time base which automatically establishes displays of a selected number of cycles, or portions of cycles, and provides the appropriate scaling information for the waveform of an input electrical signal.
In a conventional digital storage oscilloscope, input analog electrical signals are sampled at a known rate to acquire digital representations of instantaneous signal amplitudes (samples). The samples, now in the form of digital data, are stored in a memory having a finite record length which is typically equivalent to one screen width of displayed waveform. Thereafter, the digital data may be recalled from the memory, processed and converted back to analog values, and displayed at a known display-clock rate to reconstruct and replicate graphic waveforms of the original input analog signals.
A common problem associated with digital oscilloscopes is establishing a stored image of the signal of interest, particularly if the signal waveform is complex and only a small portion of it is of interest, or if the frequency of the input signal is changing. It has become commonplace to utilize the computing power available in more sophisticated oscilloscopes to analyze an input signal and provide event recognition for triggering and automatic time base adjustments to aid the user in obtaining a usable display. A setup procedure is carried out by software wherein through a time-consuming, iterative acquire, analyze and adjust procedure, the triggering and time base sweep rate are established to provide an intelligible display. In actual practice, however, such automatic setup and ranging has its failures and shortcomings. For example, in addition to being very slow and time consuming, this approach for use in digital storage oscilloscopes is prone to errors and may miss signals entirely, particularly if the amplitudes and frequencies of the input signal are changing, or if the signals are occurring at a very low frequency or repetition rate.
One measurement situation in particular that does not lend itself to conventional automatic setup procedures is the waveform analysis of rotating machinery such as internal combustion engines because the area of interest may occur at a specific crank angle that is related to other factors. For example, in a four-stroke diesel engine wherein one complete engine cycle requires two revolutions of the crankshaft (or 720 degrees), the area of interest may be signals related to needle lift, ignition pulses, or fuel pump signals.
U.S. Pat. No. 4,399,407 to Kling et al. teaches an engine analyzer in which the period of a single engine cycle is measured, and a computer uses the measurement to vary the sample clock rate of an analog-to-digital converter (ADC) during signal or waveform acquisition so that the ADC always produces a constant number N of samples in the form of digital data. This allows relatively simple circuitry to store and display the N samples representing a single cycle in a constant screen width. For this system to operate properly, a variable clock is required, and new clock rates continually must be computed before a signal can be acquired. For the relatively slow signals of an engine analyzer, the time delays between acquired signals may be tolerated.
Other measurement situations not suitable for conventional automatic setup procedures include frequency-response and variable-phase signal testing and measurements associated with voltage-controlled oscillators and variable frequency transducers. In such situations, it would be desirable to specify the number of cycles to be acquired for viewing.
Another measurement situation that has been cumbersome and difficult in the past is measuring degrees along the horizontal axis. For example, The typical method of measuring the injection timing of internal combustion engines is to put markers on one trace which represent the angular position of the crankshaft. Such markers are manually aligned with graticule scale lines on the oscilloscope display by adjusting the variable time base and horizontal position controls. The oscilloscope user must mentally convert the scale to a number of degrees per division to interpret the measurement. A small change in engine speed means the manual screen calibration must be repeated.
What is desired is a general purpose digital storage oscilloscope capable of acquiring and displaying input signals over a wide range of frequencies, and automatically establishing, by menu setups or user selection, any of a number of cycles of an input signal for viewing, and an oscilloscope that is capable of rapidly adapting to changes in signal frequency automatically in order to maintain a fixed display despite the changes. Additionally, for such applications, it would be desirable to provide an oscilloscope horizontal display axis which could be expressed in degrees per division as well as time per division.